Improvement of CANDU safety parameters by using CANFLEX

Abstract

The Power Coefficient of Reactivity (PCR) is an important safety parameter for inherent stability and safety of nuclear reactors. In the CANDU6 reactor, PCR is very close to zero unlike the typical pressurized water reactors due to the combined effect of a relatively less negative fuel temperature coefficient (FTC) and a positive coolant temperature coefficient (CTC). To ensure stability of CANDU reactors in power transient situations, the PCR should be negative for a wide range of operational power. However, PCR in CANDU6 reactor is calculated to be slightly positive when the modern reactor physics codes are used. In this study, the CANFLEX fuel design is introduced to improve safety parameters of CANDU6 using natural uranium. A CANFLEX fuel bundle has more fuel pins and it provides a smaller linear power than the standard one. Additionally, the bundle power distributions are flattened in CANFLEX by introducing two types of fuel elements. As a result, the average fuel temperature is significantly lower. Furthermore, the coolant flow area is slightly larger than in the standard 37-lement bundle. Due to the design changes, CANFLEX is expected to improve the FTC and CTC. Based on an efficient 2-D lattice method, safety parameters of the CANDU6 reactor have been evaluated by using a continuous energy Monte Carlo code, Serpent, for both the standard 37-element and CANFLEX fuels. For determination of 2-D lattice models representing an equilibrium CANDU6 core, Monte Carlo depletions are performed for various lattice models. For accurate Monte Carlo simulations, a huge number of neutron histories are used in all calculations, and the thermal motion of U-238 is also considered with the DBRC (Doppler Broadened Rejection Correction) method for accurate evaluation of FTC. A continuous FTC has been determined for the two fuel types and CTC is characterized for a range of coolant temperature. The PCR is also evaluated for a wide range of reactor power. In order to achieve...